It is desired for wired and wireless communication systems to be as efficient as possible to maximise the numbers of users that can be served and the data rates used by the users in the system. Also, the systems should be robust such that data can be transmitted also in bad conditions, such as bad radio conditions for a wireless communication system.
Orthogonal Frequency Division Multiplexing (OFDM) is an example of a modulation scheme or method that can be used in a communication system for efficiently and robustly transmitting data over a channel. The basic principle of OFDM and similar modulation schemes is to split a high rate data stream into a number of lower rate data streams that are transmitted simultaneously over a number of subcarrier frequencies, i.e. subchannels. In this way, the signals, i.e. waveforms, of the lower rate data streams are superimposed into an OFDM signal that is transmitted. To obtain a high spectral efficiency the frequency response of the subchannels are overlapping and orthogonal, hence the name OFDM. By introducing a cyclic prefix as a “guard time” between each packet data unit (in OFDM called OFDM symbol) of the OFDM signal this orthogonality can be maintained even though the signal passes through a time dispersive channel. The cyclic prefix is a copy of the last part of the OFDM symbol and it is inserted before the symbol. This makes the transmitted flow of OFDM symbols periodic, and it plays a decisive role in avoiding intersymbol and intercarrier interference. OFDM is described in e.g. “An introduction to orthogonal frequency-division multiplexing” by Edfors et al, Research Report TULEA 1996:16, Div. of Signal Processing, Luleå University of Technology, Luleå, September 1996. OFDM is also described in “OFDM for Wireless Multimedia Communications” by Prasad et al, Artech House, 2000, ISBN 0-89006-530-6. OFDM is a modulation scheme used in Digital Audio Broadcasting (DAB), Digital Video Broadcasting (DVB), the Wireless Local Area Network (WLAN) standards Hiperlan2 and IEEE 802.11a, the Wireless Metropolitan Area Network (WMAN) standard IEEE 802.16 and ADSL (Asymmetrical Digital Subscribes Lines). It is also an envisioned modulation scheme for a future 4th Generation radio interface for mobile communication.
One of the drawbacks with OFDM is that, for an OFDM signal that forms an OFDM symbol, the ratio between the peak power and the average power (peak to average ratio, PAR) may be large. This may occur since the waveforms of the subchannel data streams are superimposed into an OFDM signal, and, the waveforms may in a certain moment add up to a momentarily high peak value. This large peak to average ratio reduces the efficiency of the power amplifier in the transmitter because the power amplifier has to be designed with large back off. If not, there will be distortions on the signal for occasions with high signal power. I.e. there is a trade off between distortion and power efficiency. The higher PAR value, the more sensitive the OFDM symbol will be to non-ideal characteristics of the transmitter, for example in the power amplifier. Other multicarrier modulation schemes experiencing similar problems may be e.g. Orthogonal Code Division Multiplex (OCDM) or Multi Carrier Code Division Multiple Access (MC-CDMA). As shown, there exists a need for a system which, when used for transmission of packet data units represented by waveforms, minimizes distortion due to high sensitivity of the packet data units to non-ideal transmitter characteristics. More particularly, there exists a need for a system which, when a multicarrier modulation scheme is used for transmission of packet data units, maximizes power efficiency while at the same time minimizing distortion due to high sensitivity of the packet data units to non-ideal transmitter characteristics, such as high PAR values.
This need may be taken care of by designing efficient linearization techniques for power amplifiers such that the power amplifiers may amplify higher signal powers without distortion, such as Dorothy amplifiers, LANC (Linear Amplification with Non-linear Components) and feed forward compensation mechanisms. However, such amplifier improvements result in expensive amplifiers, consuming comparatively much power.
Several other procedures for avoiding the problems due to non-ideal transmitter characteristics, and especially due to high PAR values has been studied, as described in “OFDM for Wireless Multimedia Communications” by Prasad et al. A common aspect for many of those procedures is that they primarily strive to solve the problems due to high PAR values in the modulation and/or error correction coding process. However, other procedures based on various forms of clipping of the peaks have also been described. Although, by clipping the peaks, the signal to noise ratio is lowered. Other procedures, as described in e.g. U.S. Pat. No. 6,175,551 and U.S. Pat. No. 6,751,267, are based on peak cancellation through subtraction of an appropriate reference function or replacement symbol. A drawback of these procedures is that extra overhead is created, i.e. they are power inefficient. Moreover, in one embodiment of U.S. Pat. No. 6,751,267, intentional errors are introduced to reduce the PAR, at the expense of reduced error correction performance.
A typical feature in communication is to use scrambling of data at transmission and descrambling of the data at reception. The objective is to randomize the data stream for spectrum shaping purposes, but also to minimize the possibility of transmission of an unmodulated carrier and to ensure adequate numbers of bit transitions to support clock recovery. In e.g. Hiperlan 2, as described in “Broadband Radio Access Networks (BRAN); HIPERLAN Type 2; Physical (PHY) layer”, ETSI TS 101 475 V1.2.2 (2001-02), the physical layer receives packets from above layer (that among other things handle retransmissions) and scramble the data prior coding and modulation. The scrambling generator uses a seed that changes from MAC frame to frame. The result of the above is that, although circumstantial, Hiperlan2 uses a technique wherein if a packet was not correctly received by the receiver, and the receiver has sent a Negative Acknowledge message to the transmitter, the transmitter will re-scramble the packet before it is retransmitted. As a result, a differently coded packet with probably less sensitivity to non-ideal transmitter characteristics, by having a lower PAR for example, will be achieved, if the reason for the packet not reaching the receiver in the first place was that it e.g. had a high PAR. In this technique, power and transmission resources are unnecessarily used for transmitting packets that subsequently need to be retransmitted.
Despite much research effort in this area over the years, a fully satisfactory solution for maximizing power efficiency while minimizing errors in transmitted packet data units due to high packet data unit sensitivity to non-ideal characteristics of the transmitter, such as high PAR values, with a reasonable complexity has not yet been found.